The present invention is directed to a method for the removal of a molding core from a molded product after the product has been molded, and a cured core and product made thereby.
Over the years considerable attention has been directed to the development and expansion of lost core technology for the molding of products having complex shapes, undercuts or negative drafts or complex cavity configurations. In lost core technology such complex shapes and configurations which cannot typically be formed utilizing permanent, reusable molding forms are formed by using a shaped core or other mold form on a one time basis to form the portion of the product which is of complex configuration and is then removed from the complex portion of the molded product by disintegrating the core away from the molded product.
Various materials and procedures have been employed in the forming and removal of such cores, all of which have their disadvantages.
One such prior procedure involves the use of low melt metals, such as tin, bismuth or other low melt alloys. In this procedure the low melt metal is first shaped into the negative of the complex shape which is to be present in the finished molded product. This metal core is then positioned in the mold and the material from which the finished product is to be molded is pored or injected into the mold about the core. Once the material from which the product is to be made has solidified, the molded product together with the core are removed from the mold and are heated to melt the core away from the finished product.
This low melt metal procedure suffers a number of disadvantages In the first instance the procedure can only typically be employed in the molding of materials which are of a higher melt temperature than the low melt metal core material. Thus, the procedure is not generally usable in the molding of plastic polymers which have a lower melt or decomposition temperature than the metal of the core. Another disadvantage is that the heat and pressures during molding tend to deform the core. Moreover, the core material is heavy and expensive, and it may be toxic. Therefore, the low melt metal is difficult to handle and process. The low melt metal procedures are also energy intensive requiring large amounts of heat in the melting process, and they frequently require high temperature oil baths which are both expensive and hazardous. The low melt metal procedures are also difficult to control during the core removal to prevent damage to the molded end product, and the low melt metals are hard to reclaim. Still another disadvantage is that the low melt metal procedures typically require relatively long periods of time for the removal of the core which may be upwards of 45 minutes or more.
Water soluble polymers, such as amorphous acrylic base copolymers, have also been employed as core materials These water soluble polymers represent a substantial improvement over the low melt metal procedures because they are simpler to tool and they enjoy a reduction in material costs, weight and toxicity. However, they are generally only capable of use in the molding of plastic polymers because the typical melt temperatures of the water soluble polymers is about 350-410.degree. F. Moreover, the water soluble polymer core itself must typically be hollow to allow the water to enter the core which is to dissolve the core for removal. Thus, the core must usually be formed by fusing two pieces together with the attendant disadvantages of fusing and positioning of the core parts for fusing. Also because the core is hollow, it is not as strong as a solid core would be. The water soluble polymer cores also require a considerable time for removal of 15-20 minutes or more, and they are difficult to preheat due to their relatively low melt temperature and the possibility of flexing or distortion of the core. Another disadvantage of the water soluble polymer cores is their relatively high cost, although they may be reclaimable upon removal.
Polymer cores have also been utilized which are removable with chemical solutions or acids. These cores also suffer the low temperature melting and relatively high expense disadvantages, and they are not usually subject to reclamation. Moreover, the chemical solvents or acids present their own disadvantages in handling, storage, expense and disposal.
Cores have also been utilized for the molding of high temperature materials, i.e. metals, in which the core is formed of sand which is bound into its discrete desired configuration by binders including sodium silicate and a sugar, such as dextrose. In these sodium silicate-dextrose bound cores, once the molded product has been formed, it and its core are removed from the mold and vibrated to disintegrate the core. The purpose of the sugars in these vibrationally removed cores is to provide a component material in the core which will decompose when exposed to the high molten metal temperatures during molding of the products to weaken the core as the metal is solidifying so that the core will more rapidly disintegrate when subjected to the subsequent vibration.
The principal disadvantage of these vibrationally removed cores is the expense and power consumed in imparting the vibrational energy to the molded product and its core and the core, once it is removed, is difficult to reclaim due to the presence of the sodium silicate and the core fragments which may still be bound together to some extent. Moreover, the application of vibration is not well suited to plastic molded products due to the undue stresses which must be imparted to the plastic during vibration and the reduced impact qualities in the lighter more resilient plastic products as opposed to metal products. Such reduced impact qualities extend the time needed for core removal. Indeed, even the use of vibratory techniques for removal of the core from metal molded products may take considerable periods of time.
In the present invention, most if not all of the aforementioned disadvantages are avoided. In the method of the present invention, a core may be employed with equal facility for the molding of product materials which range over a wide range of melt temperatures, including ferrous metals of high melt temperatures at the high end of the range to low melt plastic polymers, such as polyethylene, at the low end of the range. In the method of the present invention, the cores are of a relatively light weight, are easily handled and inexpensive, and the core material is easy to reclaim and reuse following removal. In the method of the present invention, temperature control is simple and the core forming and removal is not energy intensive. Moreover, materials employed in the method of the present invention are neither toxic nor do they present environmental concerns. Significantly, in the method of the present invention, the core may be easily and rapidly removed in most cases in less than a minute from the molded product simply by immersing the molded product and core in a plain water bath. In the method of the present invention, the core may be either solid or hollow, but core parts need not be fused as in other prior procedures. Thus, the core is simple to form and shape, is strong and is stable in configuration. Still another advantage of the method of the present invention is that machining and finishing operations on the core are eliminated and the core may be used for the forming of either interior or exterior complex product surfaces, is dimensionally stable and is able to withstand high pressure and in many instances high temperature injection molding procedures.
In one principal aspect of the present invention, a method of removing a molding core from a molded product is provided in which the core comprises a particulate inert material which is formed into a discrete configuration conforming to the configuration of at least a portion of the molded product The particulate inert material is bound in that configuration by a cured binder comprising a water soluble carbohydrate The method comprises exposing the bound core in the molded product to water to disintegrate and remove the core from the molded product after the product has been molded.
In another principal aspect of the present invention, the binder may also include a silicate, preferably an alkali earth metal silicate, and more preferably sodium silicate.
In still another principal aspect of the present invention, the water soluble carbohydrate is a saccharide or starch.
In still another principal aspect of the present invention, the binder is cured by heating, preferably by microwave energy.
In still another principal aspect of the present invention, the particulate inert material is selected from the group consisting of sand, metal shot, plastic polymers, glass, alumina, clays and mixtures thereof.
In still another principal aspect of the present invention, the water employed to disintegrate and remove the core is heated, preferably to less than about 100.degree. C.
In still another principal aspect of the present invention, the core and molded product are immersed in a bath of the water to disintegrate and remove the core.
In still another principal aspect of the present invention, the water employed to disintegrate and remove the core is steam.
In still another principal aspect of the present invention, the molded product is formed of a material selected from the group consisting essentially of plastics and metals.
These and other objects, features and advantages of the present invention will be more clearly understood upon consideration of the detailed description of the preferred embodiment of the invention which will be described to follow.